1. Field of the Invention
The present invention relates to a display device including a capacitance-type touch panel, and to a driving method for the display device.
2. Description of the Related Art
Display devices equipped with capacitance-type touch panels, such as an i-Phone (registered trademark) and an i-Pad (registered trademark) made by Apple Inc., have been more and more widely used in recent years. In a capacitance-type touch panel, a detection output is generated due to a difference in electrode coupling capacitance between when a finger is not touching and when a finger is touching the panel.
A typical example of a related-art display device equipped with a capacitance-type touch panel will be described below with reference to FIG. 19. FIG. 19 is a schematic sectional view illustrating a fundamental structure of a related-art liquid crystal display device equipped with a capacitance-type touch panel. As illustrated in FIG. 19, a liquid crystal display device 101 of the related art is constituted by successively stacking a liquid crystal panel (LCD: Liquid Crystal Display) 110, a touch panel (TP) 120, and a cover glass 130. The touch panel 120 is disposed above the liquid crystal panel 110 with an air gap interposed between them. When an observer's finger 102 touches the cover glass 130, a capacitance is formed between the finger 102 and an electrode in the touch panel 120.
Furthermore, cost reduction, a thinner module, lighter weight, and a more sophisticated function have recently been demanded for touch panels, and an integral (In Cell) structure of a touch panel and a display device has been developed to realize those demands. In the future, it is estimated that, for example, a touch panel integrated with a liquid crystal display device will be constituted without requiring an air gap between the touch panel and the liquid crystal panel, and that a distance between a touch function layer of the touch panel and the liquid crystal panel will be shorter.
When the capacitance-type touch panel is constituted in the integral (In Cell) structure as described above, the touch panel is strongly affected by noise generated by the liquid crystal panel. In other words, when the touch panel performs a detection operation while the liquid crystal panel displays an image, radiation noise generated from the liquid crystal panel adversely affects detection sensitivity of the touch panel. In view of such a situation, a technique for reducing the influence of noise from the liquid crystal panel is developed to prevent degradation in performance of the capacitance-type touch panel.
According to the technique disclosed in Hiroshi Haga et al. (“Touch Panel Embedded IPS-LCD with parasitic Current Reduction Technique”, SID 2010, pp. 669-672), for example, a frame is time-divided such that the touch panel is driven during a period in which the liquid crystal panel is in a state not driven for display (i.e., during a period between frames in which any display operation is not performed), thereby reducing noise received by the touch panel from the liquid crystal panel.
However, with the technique disclosed in Haga et al., there is an upper limit in a sensing time of the touch panel because of the necessity of securing a display period (about 14 ms) of one frame. Accordingly, a level of a signal detected by the touch panel is reduced and the performance of the touch panel cannot be effectively improved.
Furthermore, with the technique disclosed in Haga et al., because the touch panel is driven by utilizing the period between frames in which any display operation is not performed, the frequency of a coordinate output in the touch panel is limited to 60 Hz. For example, in order to detect a high-speed input entered with a handwritten input using a pen or the like, the frequency of the coordinate output is generally required to be no lower than 180 Hz to 240 Hz. Thus, the technique disclosed in Haga et al. is not adaptable for such high-speed driving of the touch panel.